Severe reactor accidents

Schuster, E. Fission product release and dispersion in LP-FP-1. Proc. Open Forum The OECD LOFT Project Achievements and Significant Results. Madrid 1990, p. 224-241 Stephenson, W., Dutton, L. M. C., Handy, B. J., Smedley, C. Realistic methods for calculating the release and consequences of a large LOCA. Report EUR 14179 EN (1992) Sunder, S., Shoesmith, D. W., Christensen, H., Miller, N. H. Oxidation of UO2 fuel by the products of gamma radiolysis of water. J. Nucl. Materials 190, 78—86 (1992) 

US Nuclear Regulatory Commission (NRC) Technical bases for estimating fission product behavior during LWR accidents. Report Nureg-0772 (1981) 

Voilleque, P. G. Measurements of radioiodine species in samples of pressurized water reactor coolant. Nucl. Technology 90, 23-33 (1990) 

Models of iodine behavior in reactor containments. Report ORN17TM-12202 0992) 

Wilhelm, J. G. Iodine filters in nuclear installations. Report Commission of the European Communities, Contract No. 1175-80-12 LA (1982) 

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The ingestion dose contributes very little to the dose from a severe reactor accident and is usually not computed. However, the food pathway is a major determinant of bow the exposed area must be treated in the months and years following the accident. If the ground concentration is high, the land may be interdicted from agricultural u.se or grazing. 

Severe reactor-accident release, waste-repository release, land disturbance, and others, if hydrogen is produced via uranium feedstock ... 

PLYS, M.G., Hydrogen Production and Combustion in Severe Reactor Accidents An Integral Assessment Perspective, Nucl. Tech. 101 (1993) 400-410. 

There is an enormous number of radioanalytical procedures based on solvent extraction and here it is only possible to give a few examples. The examples chosen have been taken from the analysis of samples from the European PHEBUS project performed at the Nuclear Chemistry, Chalmers University of Technology. Very briefly, the Phebus reactor was used to study the products formed in severe reactor accidents. The released gases and aerosols were collected in filters and in water at different positions in the experimental setup. 

The upper, open-ended band represents low-probability severe reactor accidents with consequences beyond the facility boundary. It is not expected that any off-site evacuation of the public would be needed under any circumstances for small power reactors, however, minor releases of radioactivity within the specified regulatory limits may occur. The accident response team would secure the site, restore a safe, contained shutdown state, and perform any required cleanup. 

As a result, the plant should be simpler to operate and maintain than present day LWR plants, and the elimination of severe reactor accidents as a practical concern should also contribute to simplified operation, making it possible for the plant operator(s) to concentrate on producing electric energy. It can also be noted that PIUS compares favourably against the top-tier requirements of the ALWR Requirements Documents developed by the Electric Power Research Institute (EPRI) in the US. 

Radionuclides are released to the containment as gases and as aerosol particles by a variety of processes during severe accidents. Modem, mechanistic analyses of these radionuclide releases and the subsequent behaviour of aerosols and vapours under reactor accident conditions strive to be realistic. This realistic approach contrasts with the deliberate attempt to be conservative (which may not have been successful) in the definition of radionuclide behaviour for the design of nuclear power plant safety systems. A discussion of the various radionuclide release processes during severe reactor accidents is presented in Chapter II. Of primary interest in these discussions of release is the potential magnitude of radionuclide release and the radionuclides of most concern. Factors that most affect radionuclide release but can also be affected by accident management measures are discussed. 

These documents provide information on severe reactor accidents at two WER type plants. 

These experimental studies illustrate the important consequences of air ingression into the reactor vessel when hot fuel residues are present. Ruthenium releases as well as the releases of some other radionuclides are greatly accentuated. The proceedings of the seminar provide also a good digest of much of the world-wide research on severe reactor accident source terms at the time. 

S-21. D.A. Powers, L.N. Kmetyk, and R.C. Schmidt, A Review of the Technical Issues of Air Ingression During Severe Reactor Accidents, NUREG/CR-6218, SAND94-0731, Sandia National Laboratories, Albuquerque, NM, December 1994. 

L.E. Herranz and F. Robledo, A Potential Strategies to Control Iodine Released into the Containment in the Case of a Severe Reactor Accident , In the Proceedings of OECD Specialist Meeting on Selected Containment Severe Accident Management Strategies,... 

In the second case, only gamma dose-rate instruments and operational intervention levels are needed to determine where actions are needed. The first category of accidents, however, is much more difficult to assess. For such events, protective action must be taken before or shortly after a very severe release to be effective in reducing the risk of severe deterministic health effects near the facility. The strategy for taking protective actions for severe reactor accidents is discussed above under Public Protective Actions. 

FIGURE 9.7 U.S. NRC protective actions for severe reactor accidents. (Source NRQ 1996)... 

Rogers, J.T., D.A. Meneley C. Blahnik, VC. Snell, and S. Nijhawan. 1995. Coolability of severely degraded CANDU cores. In International Seminar on Heat and Mass Transfer in Severe Reactor Accidents, CESME, May, Turkey. 

In addition to the radionuclides produced in the nuclear fuel, others are generated in the core structural materials such as the fuel rod cladding, spacers, fuel assembly end pieces, control rods, and core support materials. Here, the main production mechanism is neutron activation. A fraction of the radionuclides generated in this manner is also released to the primary coolant during steady-state operation of the plant or to the primary circuit in the course of a severe reactor accident. Therefore, the following discussion will also cover these radionuclides. 

The two first-mentioned reactions of CsOH are of little significance in a reactor accident, since only small amounts of these materials are present in the primary circuit outside the reactor pressure vessel. The reaction with stainless steel surfaces, however, may lead to significant retention of cesium in the reactor pressure vessel, in particular at the upper core structural materials there will be a more detailed discussion of these reactions in the context of severe reactor accidents (see Section 7.3.2.). 

Figure 7.2. Temperature regimes for liquid-phase formation in a severe reactor accident and possible consequences for reactor core damage...

Due to the differences in design, in BWR plants other initiating events for severe reactor accidents have to be considered than in PWR plants the essential aspect common to them is insufficient decay heat removal from the reactor core. Out of the number of possible events, two different accident sequences can be selected which, in a BWR of the KWU series 90, wUl cover the other sequences as regards their consequences (Fabian and Eyink, 1988). These sequences are ... 

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